1. Field of the Invention
The present invention concerns carbon fibers suitable to use in electroconductive composite materials, etc.
2. Description of the Prior Art
Since carbon fibers are light in weight and excellent in mechanical strength, as well as have satisfactory electroconductivity, they have been utilized in various application fields of use as composite materials in combination with metals, plastics or carbon materials.
By the way, since the electroconductivity of the carbon material is poor as compared with that of metal material, improvement in the conductivity of the carbon material has now been put under study, and it has been known to introduce various kinds of molecules, atoms or ions, for example nitric acid, between the layers of graphite crystals thereby obtaining an inter-metallic compound with improved conductivity. Further, while it has been considered that the intercalated compound of a covalent bond type obtained by reacting graphite and fluorine exhibits insulative property, it has also been known that an electroconductive intercalated compound can be obtained by reacting flaky or powdery graphite such as natural graphite or artificial graphite and fluorine. However, since such an intercalated compound is powdery, it involves a problem that homogenous and stabilized conductivity cannot be obtained with ease and the strength is reduced when the compound is formulated into a composite material.
On the other hand, because pitch type graphite fibers or PAN type graphite fibers show not so well developed crystal structure, excellent electroconductivity cannot be obtained for the intercalated compound and, in addition, it is difficult to attain uniform dispersion thereof in the composite material Furthermore, it has also been known to use graphite fibers obtained by graphitizing gas phase grown carbon fibers having more complete crystal structure and reacting them, for example, with nitric acid, metal chlorine or bromine, even though they involve drawbacks such as that the stability is poor, to increase electric resistance with lapse of time and they bring about corrosion to the apparatus in contact therewith due to decomposition products.